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Final results of the election are expected on Monday

Public broadcaster NHK said Mr Abe's Liberal Democratic Party (LDP) and its junior coalition partner, the Komeito party, would between them take 67 to 76 of the seats available.

The coalition already controls 77 seats of the other half of the upper house.

Much more than Abenomics

The prime minister fought his campaign on his economic record, but the sub-text of the election was the power to amend the constitution, the BBC's Stephen Evans, in Tokyo, said.

Mr Abe is thought to want to change Article 9, the so-called pacifism clause which forbids Japan from fighting wars abroad. It was imposed by the US after Japan was on the losing side in World War Two, 70 years ago.

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Half the seats in the upper house of parliament were up for grabs

Some in Japan view the constraint as unfair, our correspondent says, and the rise of China has reinforced the view on the right that the clause should go.

But, in a TV interview as the votes were still being counted. Mr Abe said he was in no hurry to address the issue.

"I have two more years to my term [as LDP president] and this is a goal of the LDP, so I want to address it calmly."

The opposition has asked voters to reject any adoption of a more assertive military role.

Mr Abe also said the election result was a vote of confidence on his economic policies, although he has admitted himself that his Abenomics, aimed at ending debilitating deflation, are only "half done".

"We were given approval for our mandate to powerfully pursue Abenomics. We would like to continue with our efforts to achieve what we have promised," he said.

This was the first nationwide election since the voting age was lowered from 20 to 18.